CHAPTER 10
MECHANICAL SEPARATIONS

In sedimentation, two immiscible liquids, or a liquid and a solid, differing in density,are separated by allowing them to come to equilibrium under the action of gravity, the heavier material falling with respect to the lighter. This may be a slow process. It is often speeded up by applying centrifugal forces to increase the rate of sedimentation; this is called centrifugal separation. Filtration is the separation of solids from liquids, by causing the mixture to flow through fine pores which are small enough to stop the solid particles but large enough to allow the liquid to pass. Sieving, interposing a barrier through which the larger elements cannot pass, is often used for classification of solid particles.

Mechanical separation of particles from a fluid uses forces acting on these particles. The forces can be direct restraining forces such as in sieving and filtration, or indirect as in impingement filters. They can come from gravitational or centrifugal action, which can be thought of as negative restraining forces, moving the particles relative to the containing fluid. So the separating action depends on the character of the particle being separated and the forces on the particle which cause the separation. The important characteristics of the particles are size, shape and density; and of the fluid are viscosity and density.